The present invention relates to thermotropic liquid crystalline aromatic polyesters.
More particularly, the present invention relates to thermotropic liquid crystalline aromatic polyesters easily processable in the molten state and having the mesogen group in the main chain.
Thermotropic polyesters showing optical anisotropy in the molten state are known products and are described in many examples in literature such as British Polymer Journal (December 1980), page 154: "Liquid Crystal Polymer"; Journal of Macromolecular Science-Chemistry (1984), page 1705: "Liquid Crystalline Aromatic Polyesters"; Die Angewandte Makromolekulare Chemie (1982), 109-110 page 1 "Rigid Chain Polymers"; Die Angewandte Makromolekulare Chemie (1986), 145-146, page 231: "Thermotropic Liquid Crystalline Polymers"; Journal of Molecular Science Review (1986) C26(4), page 551; "Liquid Crystalline Polymers: a novel state of material".
The use of these polyesters allows one to obtain fine fibers exhibiting high thoughness or molded manufactured articles, for instance those articles obtained by injection molding, having suitable rigidity, hardness and toughness.
Generally, the polymers endowed with the above mentioned characteristics are easily processable in the molten state, highly resistant to heat and oxidation and furthermore, because of their crystallinity, these polymers show high HDT (Heat Distortion Temperature) and resistance to solvents.
The foregoing polymers can be obtained by polycondensation for example, of terephthalic acid and a 4,4'-dihydroxydiphenyl melt but at temperatures too high to be easily processed. Methods used to lower the melting point suggest using more substituents on the aromatic rings of diphenol or of terephthalic acid or by modifying the polymer by the addition of other comonomers.
U.S. Pat. No. 3,637,595 describes the preparation of thermotropic liquid crystalline polyesters by polycondensation of 4,4'-dihydroxydiphenyl, terephthalic acid and p-hydroxybenzoic acid. The thus obtained polymer, although exhibiting high mechanical characteristics and resistance to oxidation and solvents, possesses a high melting point, higher than 420.degree. C. and therefore it is processable only with difficulty or, at any rate, requires unconventional apparatus when injection molding the polymer.
In U.S. Pat. No. 3,975,487 the above mentioned drawback has been partially overcome by using bent comonomers, such as isophthalic acid, the presence of which however, if in high quantities, can cause the destruction of the liquid crystalline properties of the finished polymer.
In Journal of Polymer Science : Polymer Physic Edition vol 21, 119, the preparation of polyesters starting from 4,4'-dihydroxydiphenyl and different aliphatic diacids is described; however, these polymers, because of the presence of compounds with aliphatic chain, exhibit little interesting physicomechanical properties.
U.S. Pat. No. 4,617,370 describes the preparation of liquid crystalline polymers starting from 4,4'-dihydroxydiphenyl substituted in the 3,3', 5,5' positions by phenyl or alkyl or alkoxy radicals containing a lower number of carbon atoms and substituted in at least one of the positions 2,2',6,6' by a halogen atom.
This monomeric unit is allowed to react with dicarboxylic aromatic acids and the polymer thus obtained has a melting point in the range between 270.degree. and 250.degree. C.
However, according to what is described in this patent, it is possible to obtain a polymer endowed with liquid crystalline properties only if the hydroxyaromatic acid is used in the polymerization mixture, otherwise the polymer obtained shows an isotropic molten state.
In European Pat. Appln. No. 201,831 thermotropic liquid crystalline polyesters, obtained by polycondensation of a bicarboxylic aromatic acid of 4,4'-dihydroxyphenyl can be used for the preparation of thermotropic liquid crystalline polyesters either in the non-substituted form together with other comonomers, or in the substituted form selecting suitable substituent groups.